As is known in the art, there exists a class of materials referred to as thermoelectric materials. A thermoelectric material is a type of material which can directly convert thermal energy into electrical energy or vice versa.
Although certain thermoelectric materials have been known in the art for a number of years (e.g.--bulk semiconductors), it has only recently been found that thermoelectric materials having a superlattice structure can possess thermoelectric properties which are better than the corresponding thermoelectric properties of other thermoelectric materials.
A superlattice structure denotes a composite structure made of alternating ultrathin layers of different component materials. A superlattice structure typically has an energy band structure which is different than, but related to, the energy band structures of its component materials. The selection of the component materials of a superlattice structure, and the addition of relative amounts of those component materials, will primarily determine the resulting properties of a superlattice structure as well as whether, and by how much, those properties will differ from those of the superlattice structure's component material antecedents.
It is generally known that thermoelectric materials and thermoelectric materials having a superlattice structure find application in the fields of power generation systems, and the heating and/or cooling of materials. One problem, however, is that although these fields place ever-increasing demands on thermoelectric materials to possess ever-improving thermoelectric performance characteristics, the thermoelectric materials and thermoelectric materials having a superlattice structure known in the art have, as of yet, not been able to keep pace with such performance demands.
One way to predict the thermoelectric behavior of thermoelectric materials or thermoelectric materials having a superlattice structure in the fields of power generations systems, and the heating and/or cooling of materials is to calculate a thermoelectric figure of merit for the materials. The thermoelectric figure of merit, ZT, is a dimensionless material parameter in which T corresponds to temperature and Z is the figure of merit. ZT is a measure of the utility of a given thermoelectric material or thermoelectric materials having a superlattice structure in power generation systems, and heating and/or cooling applications at a temperature T.
The relationship of ZT to the material properties of thermoelectric materials and thermoelectric materials having a superlattice structure is shown by the following equation: EQU ZT=S.sup.2 .sigma.T/.kappa.=S.sup.2 ne.mu.T/(.kappa..sub.1 +.kappa..sub.e)=P.sub.F T/.kappa.=S.sup.2 GT/K
in which S, .sigma., T and .kappa. are, respectively, the Seebeck coefficient, the electrical conductivity, the temperature, and the thermal conductivity and where n, e, .mu., .kappa..sub.1 and .kappa..sub.e are, respectively, the carrier density, the electronic charge, the carrier mobility, the lattice part of the thermal conductivity and the electronic part of the thermal conductivity, and where P.sub.F is the power factor, and where G and K are, respectively, the electrical conductance and the thermal conductance.
Generally, it is known in the art that it is desirable for thermoelectric materials to have a relatively high value for their thermoelectric figure of merit (ZT) in order for those thermoelectric materials to perform well in the fields of power generation systems, and the heating and/or cooling of materials. From inspection of the above equation, it appears that to provide a thermoelectric material having a high ZT, one need only fabricate on it a superlattice structure having relatively high values for its Seebeck coefficient, its electrical conductivity, and its temperature while, at the same time, having a relatively low value for its thermal conductivity.
It has proven difficult in practice to provide a thermoelectric material or a thermoelectric material having a superlattice structure that has a high thermoelectric figure of merit (ZT) value. Past findings in the art have suggested that the inherent interrelationships between the material properties included in the above equation for ZT such as carrier mobility, lattice thermal conductivity, power factor and Seebeck coefficient may limit, or place a ceiling upon, the ZT values of thermoelectric materials or thermoelectric materials having a superlattice structure.
For example, thermoelectric materials such as Bismuth Telluride (Bi.sub.2 Te.sub.3) and Bismuth-Antimony (BiSb) have been used in the art to provide multistage thermoelectric modules (i.e. modules composed of two or more stages of thermoelectric materials, each stage being designed to maximize ZT over a specific temperature range). Bi.sub.2 Te.sub.3 and BiSb module materials, however, are incapable of efficient heat removal below a temperature of approximately 180.degree. K., and have a thermoelectric figure of merit (ZT) less than one. Thus, multistage thermoelectric modules do not have the desired efficiency to be useful in the wide range of temperatures encountered in many thermoelectric applications.
Another type of thermoelectric material known in the art is provided from lead selenide-telluride (PbSeTe) alloys. While the thermal conductivities of such alloys are low and thermal conductivity, as shown in the above equation, is inversely proportional to ZT, any resulting improvement to the ZT value of PbSeTe alloys as a result of their low thermal conductivities is offset by their relatively low carrier mobilities.